The semi-tandem propeller according to this invention arises from modifications of a conventional tandem propeller to achieve significantly improved operating efficiency.
Based on research performed at the David W. Taylor Naval Ship Research and Development Center, Bethesda, Md. it was determined in one case that the open-water characteristics of a tandem propeller with twice the number of blades of a single propeller is approximately 4% higher than that of a single propeller. For the same thrust loading, there is less kinetic energy in the slipstream which may explain the higher efficiency of a tandem propeller.
More uniformity of propeller loading over the whole propeller disk can be obtained by increasing the number of blades of a single propeller. Theoretically, in an inviscid fluid, optimum propeller efficiency increases with an increase in the number of blades. For purpose of illustration, when the advance coefficient is equal to 0.3 and the ideal thrust coefficient is equal to 1.0, the relationship between the number of blades and the ideal propeller efficiency is substantially as follows:
______________________________________ No. of Blades Propeller efficiency (ideal) ______________________________________ 2 .60 3 .68 4 .70 6 .73 8 .75 .infin. .78 ______________________________________
The reason that ideal propeller efficiency is increased with an increase in the number of blades can be understood from a simple momentum theory. To produce a given thrust, the propeller efficiency depends upon the induced axial velocity distributed over the propeller disc defined by the area circumscribed by the revolving blades. If the axial velocity induced in the fluid is a constant over the whole disk area, the least kinetic energy is left in the slipstream and, thereby, optimum ideal efficiency is obtained. A finite number of blades achieve a constant induced velocity only along each blade. The uniformity of velocity over the area of the whole propeller disk can be improved by increasing the number of blades.
The theoretical advantage of increased efficiency by increasing the number of blades is, unfortunately, not assured in practical applications, where it has been found in viscous fluids, such as water, for example, that an increase in blades may actually result in efficiency loss. As the number of blades increase, the passages between the blade is constricted, particularly near the hub. Water velocity through these passages has to be increased, and, as a result, blade viscous drag increased. Another problem encountered with conventional tandem propellers is that loading on forward and aft propeller blades is not equalized because they operate in different inflow fields since one is axially ahead of the other. Since propeller loading is mainly carried over the outer portion of the disk area, optimum setting of the blade outer portions, including their substantial alignment in a common plane of propeller rotation, is desirable. This may be achieved by the semi-tandem propeller configuration described herein.